The designer of a postage meter, also called a franking machine, faces many competing pressures. First and foremost is the requirement that the design of the meter satisfy the postal authorities in the country where the meter is to be used. If a manufacturer has the goal of marketing the meter in not one but several countries, the practical consequence is that the meter has to satisfy all of the postal authorities in all of the countries, or must at least be easily and inexpensively adapted to comply with the requirements in each country. Generally there will be a requirement that critical portions of the meter (for example, the ascending or descending register and the printing mechanism) be contained within a single secure housing. The housing has to be made so that it is easy to detect tampering (and attempted tampering) by visual inspection. The postage impression placed on the mail piece by the meter has to be of a printing technology that is not easily counterfeited, replicated, or altered without detection; this tends to rule out most printing technologies other than the use of formed metal print elements within a rotating print head or rotor. Indeed the vast majority of postage meters in use worldwide have print rotors using metal print elements having raised indicia which receive ink on the raised areas thereof from an ink roller, and the rotor is rotated so that the indicia come into contact with the mail piece to form the postage impression on the mail piece. In this way a relief printing impression is accomplished that is much more difficult to counterfeit than, say, a pin-matrix or ink-jet impression. The indicia include fixed portions such as the country and meter identification as well as variable portions such as the date and postage amount. The variable portions are varied through the use of print wheels that can be rotated so that particular indicia are positioned for printing.
It will be appreciated that the mechanism for the setting of print wheels represents a particularly important aspect of the meter design. To be approved by the postal authorities, the mechanism has to provide a highly reliable linkage between the print wheels and the ascending and/or descending register, so that it is highly likely that any printing of postage will result in an accurate debiting of postage value from the descending register and/or accurate creding of postage value to the ascending register. In a pure mechanical postage meter the linkage is, of course, a mechanical linkage between one or more mechanical registers and the print wheels. In a postage meter having electronic registers the linkage is accomplished by a blend of hardware and software.
To satisfy postal authorities the linkage between the print wheels and the registers must not only be highly reliable over hundreds of thousands of postage impressions, it must also be quite robust against a variety of harms including abuse by users. Where the linkage is based partly in software it is typically required that the software monitor the mechanism so that malfunctions are detected and annunciated.
The linkage between the print wheels and the registers is difficult to design not only because of the above-described regulatory requirements but also because it has to function in the context of a rotating print rotor. The rotor has to be capable of rotating a million times or more in the life of the meter, which emphasizes the fact that the rotor must be freely rotatable relative to the rest of the meter. Somehow the designer must arrive at a way to accomplish the print-wheel linkage across the boundary between the rotor and the rest of the meter. The linkage has to provide its highly reliable function of adjusting print wheels when the rotor is not moving, and yet has to lock the print wheels during times when the rotor is moving.
The patent art is filled with approaches that have been proposed for the linkage between the print wheels and the registers, few of which have been commercially successful. One successful approach is that employed in the F310 postage meter. In that prior art approach, the print rotor has a long axle with an H-shaped cross section. Along the axle and set into recesses of the axle are racks that slide along the axle. Each rack is linked to a respective print wheel in the rotor, so that in a typical rotor with four or five print wheels there will be a corresponding number of racks along the axle. When the rotor is in a "home" position, the racks engage with gears in the main body of the postage meter. When the rotor is out of the home position (generally because a postage printing operation is in progress) the racks are no longer in engagement with the gears in the main body of the postage meter.
What has been described thus far is the linkage from the print wheels to gears in the main body of the postage meter.
Continuing the account of a prior-art way to link the print wheels to the registers, what will now be described is a prior-art way to link the gears in the main body of the postage meter to the registers. But first it is instructive to review some of the constraints on the designer regarding this part of the linkage. The gears have to be under the complete control of the microprocessor, located in the main body of the postage meter, which maintains and updates the registers. The microprocessor has to be able to actuate electrically controllable elements such as motors and electromagnets to bring about any desired position of the print wheels. It has to be able to receive information from electrical sensors such as Hall-effect sensors, LED-phototransistor pairs, or mechanical electric switches, to have highly reliable information about the positions of the print wheels. All of this has to be accomplished in a highly reliable way (to satisfy the postal authorities) so that any of a wide range of failure modes and tampering attempts will be detectable by software so that appropriate action may be taken, such as disabling the meter or at least logging the suspicious event. And it has to be accomplished in a way that continues to work properly after several hundred thousand setting and printing cycles.
Beyond the requirements imposed by the postal authorities are practical requirements. The manner in which the microprocessor controls the gears (that engage the rotor axle racks) cannot be too bulky, the parts count cannot be permitted to be too high, the parts cannot be unduly expensive, and the machine should be easy and fast to assemble. What's more, from the user's point of view the setting has to take place fairly quickly.
Conceptually, one way a microprocessor could control such gears could be as follows. For each gear that needs to be controlled, a stepper motor is provided. The stepper motor is linked in a reliable way to the gear, and a position sensor is also linked in a reliable way to the gear. The sensor could be an encoder, so that its output at all times communicates the absolute position of the gear. With such an arrangement, when the processor wishes to bring about some particular setting of a print wheel, the processor simply reads the output of the encoder, determines whether or not the print wheel is already in the desired position, and drives the stepper motor as needed until the encoder indicates that the print wheel is in the desired position.
Such an arrangement is conceptually simple, but it has numerous drawbacks. Where five print wheels are to be controlled, this arrangement would require five stepper motors, five resolvers, and all the interface electronics attached thereto. Such equipment takes up a lot of space, costs a lot of money, and consumes a lot of power in energizing the motors. Software is also an area of concern. If the software is simple, or if the processor is not very powerful, then it might be necessary to actuate the five stepper motors one after the other. Such serial setting might take too long for the user of the meter. Contrariwise, if the five stepper motors are to be operated simultaneously, then the processor has to be more powerful and the software more difficult to write and test.
Yet another arrangement that some have proposed is the user of, say, two motors. A first motor determines which of the five print wheels is to be set, and a second motor does the setting. In a typical arrangement of this type, the first motor controls a transmission which selectively connects the second motor to one or another of the gears for the print wheels. It is all to easy to make mistakes when designing such a system; for example when one of the gears is being set some attention must be given to fixing the positions of the other gears. But at least one such system set forth in the prior art overlooks this and leaves all the gears (other than the one being set) free to move in response to any user manipulation; it would be all too inviting (to the user who is predisposed to misdoing) to attempt to get more postage than is being paid for by manipulating the print wheels in such a system.
Another potential drawback to such a serial set system is that setting of all of the print wheels might take longer than the user would prefer.
Serial setting has been employed in very large so-called "flatbed" postage meters, used by mailing houses that send out very large volumes of mail. Typically the meter is not brought to the post office for inspection and resetting, but instead the user pays and extra fee for the postal employee to come to the user's location to inspect and reset the meter. Because the meter is not moved very often, its large size and weight are not seen as a great disadvantage by the user. Most users of such meters print a large number of mail pieces for any particular value setting, so the setting of print wheels does not happen very often compared with the number of mail pieces sent through the machine; as a result even though the setting of print wheels may take a relatively long time this is not necessarily seen as a great disadvantage. The user who sends out very large volumes of mail is also unlikely to see high cost as a problem so long as the meter franks the mail quickly and reliably.
In either of the two above-described systems there has to be a way for the microprocessor to sense, with high confidence, the positions of the systems being manipulated. One choice, already mentioned, is the use of position encoders, which provide many-wire signals indicative of the absolute positions being sensed. Another choice is to use much simpler resolvers, which emit simple quadrature signals indicative of relative motion only. Such resolvers are generally used in connection with some sort of absolute indicator such as an end-of-travel switch. The subsystem being controlled (one of the five stepper motors in the five-motor system above, or one of the two motors in the two-motor system above) is caused to move in a direction that will trigger the absolute indicator. From that point onwards, the processor (or some dedicated electronics hardware associated with the resolver) counts pulses from the resolver to keep a running account of the position of the subsystem. It will be appreciated that if such resolvers and end-of-travel switches are used, then it is necessary to run the subsystems to the end-of-travel position from time to time to be sure that everything is in the position that it is supposed to be in; depending on the design of the setting system this may be required during every setting cycle. Such excursions take time and add to the length of time the user has to wait for a setting cycle to finish.
Still another prior-art way for the microprocessor to control the print wheels is set forth in European patent no. 62 376, published Oct. 13, 1982, assigned to the same assignee as that of the present application. As described in that patent, the control of gears, associated with print wheels is accomplished in a way that overcomes many drawbacks of the multiple-motor systems set forth above. The elements controlled by the microprocessor are but a single motor, and several electromagnets, one for each gear. The motor need not be an expensive and difficult-to-control stepper motor but can be an inexpensive DC motor. The sensors monitored by the microprocessor are LED-phototransistor pairs, each coupled to a slotted disk. (Franking is accomplished with the assistance an additional motor, typically an AC motor, located in a base that is separate from the secure housing of the main body of the meter.)
As set forth in that patent, a single shaft carries several clutches. Each clutch is linked by gearing or other mechanical means to one of the print wheels. The single shaft is controlled by the motor. If the motor is rotated in one direction, the result is that all the print wheels are brought to one extreme position such as zero (that is, the position that would print a "zero" on the mail piece) or a defined position below zero. If the motor is rotated in the other direction, the result is that all the print wheels are brought to another extreme position such as nine (the position that would print a "nine" on the mail piece). A sensor linked to the shaft permits the microprocessor to monitor the position of the shaft and the progress of its movement.
The mechanism described thus far is of interest only to the users whose postage printing needs would consist of repeated characters such as "1111" and "2222". As described in the above-referenced European patent no. 62 376, the electromagnets are employed to fix each print wheel when it has reached its desired position. Each electromagnet is associated with one of the print wheels, and has a pawl that, when released, drops into a tooth in the mechanism connecting a clutch to that print wheel. The clutch then "breaks loose", so that the print wheel remains fixed in position by the pawl, even if the axle keeps rotating. The setting cycle is as follows. First the motor is rotated in the direction that brings all the print wheels to their minimum position. All the electromagnets are energized. Then the motor is rotated in the direction that increases the numerical setting of the print wheels. When a particular print wheel has reached its desired position, the electromagnet is de-energized. When the electromagnet is de-energized it releases its pawl, which halts its print wheel.
The way the microprocessor knows when to de-energize a electromagnet is that a sensor linked to the associated print wheel will have emitted signals indicative of the print wheel reaching its desired position.
Eventually all of the print wheels will have reached their desired positions, all of the electromagnets will have been released, and the motor can stop moving. The setting process has been accomplished.
The clutches used in this arrangement have to satisfy several requirements. In the direction that brings the print wheels to their lowest position (the first part of the setting cycle) each clutch has to have a positive engagement that, without fail, returns each print wheel to its lowest position, despite whatever drag may exist due to friction and the like. In the direction that increases the position of the print wheels (the second part of the setting cycle) each clutch has to maintain positive engagement until such time as its associated electromagnet releases and drops its pawl. At the point that the pawl blocks movement of the print wheel, the clutch has to "break away" so that the two parts of the clutch rotate relative to each other and are no longer fixed together. The breakaway must not impose too great an impulse load onto the pawl, or the pawl and the teeth with which it engages could be damaged. After the breakaway has occurred, the clutch cannot impose too great a frictional load on the axle, since the axle has to be able to continue its movement so that other print wheels may be moved to their desired positions.
The arrangement just described, and set forth in European Pat. No. 62 376, offers many advantages over the prior art. It is not very bulky compared to a system with two, four, or five motors, does not consume much power, and is fairly easy for the processor to control. It is much faster to set than most serial setting systems.
But none of the systems discussed heretofore fully satisfies all the competing requirements that face the designer. For example, each of the systems discussed thus far has a paper path through which a mail piece passes; the paper path has numerous moving parts that require a relatively powerful motor for actuation. Generally an AC motor is used. But AC motors are heavy and take up a lot of space. From time to time it is necessary for the main body of the postage meter (the part within the secure housing) to be transported to a post office, for resetting with additional postage value or for a periodic check (in which the meter is examined for signs of tampering). To keep down the weight and size of the main body that is to be transported, some of the parts of the postage meter, such as the large AC motor, are placed in a meter base. The meter base is moved relatively rarely (i.e. when the meter is installed) and the base and the main body of the meter are designed so that the main body is readily removed from the base for the trip to the post office. Thus even if the base is heavy and bulky, this is not too great a problem since it does not have to move very frequently.
But the decision to apportion the moving parts of the meter into a base portion and a removable main body has drawbacks of its own. Dozens of moving parts have to be added to the design to facilitate mechanical linkages that are made and broken when the main body is placed on the base or removed from it. Each place in the main body of the meter where something connects to the base represents a place where someone might try to tamper with the meter to obtain free postage. Thus many additional moving parts have to be added to the design to protect against such tampering. The postage meter prior art is filled with all manner of shutters, interposers, sliding covers, and locks that exist solely to protect against what someone might try to do when the main body of the meter is separated from the meter base. These moving parts contribute to the parts count of the meter, and to its assembly cost and complexity. These moving parts also affect, in a negative way, the reliability of the postage meter. In the face of all these difficulties and drawbacks it might be thought that those working in the postage meter art would long ago have devised ways of making a one-piece meter that is light and small enough to take to the post office, and that also satisfies all the other demands placed on a postage meter to secure regulatory approval. Such is not the case.